In the granulation processes where particle growth is the main principle, control of the size distribution in the granulator and the final product is one of the key performance features. In a conventional granulation process the size distribution has therefore been controlled by screening and recycling a certain fraction of undersize and if necessary crushing and recycling a fraction of oversize as well. This makes it possible to directly control the composition of the final product. The energy and mass balance is thereafter adjusted with the properties of the fresh feed, drying, heating or cooling in order to give the right conditions for particle growth and quality of the product. The external screening, crushing and recycling is also solving the seeding requirement, as the crushing is creating new particles keeping the overall size and number of particles in the right range.
In the latest development of the fluid bed granulation, internal screening has been one of the new features in an attempt to eliminate the investment in external recycle loop with screening and crushing. The energy and mass balance can be solved with more direct means like cooling or heating of the granulator or by altering the water concentration in the melt or solution going to the fluid bed granulator. The economy of scale has increased the size of the fluid beds, and this is further opening for good methods for internal screening or classification.
Internal screening by physical means of a mesh, screen, slots etc. is difficult when the desired effect is to retain the smallest particles and moving the biggest particles towards the outlet. Physical restrictions inside the granulator are also clogging quickly due to the plasticity and stickiness of the particles.
The most obvious methods applied have tried to take advantage of the standard and obvious effects of segregation and classification principles based on air velocities and particle trajectories. All of these methods have failed to give the expected effect or have been seen as impractical and energy intensive.
Callen et. al. “Use of parallel inclined plates to control elutriation from a gas fluidized bed” presented in Chemical Engineering Science 62 (2007) 356-370, is one of several examples where the classification or elutriation is performed in the low-density phase above or outside the high-density phase in the fluid bed unit. The definition of the high-density fluid part is where the average bulk airflow alone is below the free fall velocity of the smallest granules in the bed and above the required air minimum critical air flow to create fluidization. In the low-density phase above the high-density fluid phase, it is possible to install guide systems changing the flow characteristics vs gravity and increase the bulk speed. When the bed has a bulk airflow close to the free fall velocity of the smallest granules, and when the size distribution is broad enough, there will be classical elutriation as in standard de-dusting and wind screening technologies. This effect however is found not to be sufficient for eliminating the external screening and crushing loop. Any separation outside the high-density phase has the inherent disadvantage of external screening where the smallest particles have to be brought back to the inlet side.
U.S. Pat. No. 6,851,558 B2 is describing how asymmetry, baffles and bed height can be used to obtain a horizontal classification over a full bed. The disadvantage is however that the bed has to be fully redesigned, and that the claimed classification effect will be disturbed by the bubble formation and effect of the spraying nozzles in each compartment. The limit to inclination and position of the baffles and compartments are reducing the claimed effect in practical beds.
The way the baffles are installed, is not giving a consistent effect. The small particles on the slow side of the baffle is also prone to moving towards the outlet at the bottom of the bed, and the larger particles on the fast side of the baffle will inevitably also follow the backward direction towards the inlet side. In this way the principle described in U.S. Pat. No. 6,851,558 B2 is contradicting itself.
US2011/0159180 A1 is describing a method of granulation where a cooling tube bundle is installed in the last part of the bed. The obtained effect is that the bed is receiving an internal cooling which is important for the most fluid bed granulation processes. The other effect is a minor classification effect. The size distribution of the product is different from the average in the bed and different from size distribution found on the top of the classifier. The classification effect is however not significant enough to eliminate the external screening, crushing and recycling loop.
These and other problems will be solved with a solution according to the present invention.